The Japanese Unexamined Patent Publication Nos. 1995-10334 and 1999-282143 disclose position-detecting devices. Each device comprises a light source to cast light such as infrared rays on a web and a sensor to detect the light penetrating, or reflected by, the web. The position-detecting devices determine the movement of one side of a web based on the change of the quantity of light detected by the sensor. The reference value for the position-detecting devices to determine the positional slippage of a web is the quantity of light detected by the sensor when the web is positioned in its reference position. Accordingly, it is necessary for the sensor to be calibrated for a correct reference value before the production line is started. To put it concretely, while a web is positioned in its reference position, the reference line of the sensor is put exactly on one of the right and left sides of the web for a correct reference value. The reference line of the sensor may be the center line of the sensor.
The operator applies a ruler to the sensor to put its center exactly on one side of a web based on the reading on the ruler; accordingly, there may occur an error in applying the ruler to the sensor.
A plurality of said position-detecting devices are arranged along the right and left sides of a web running through a production line, and every position-detecting device's sensor has to be calibrated for a correct reference value; thus, the calibration work takes a good deal of labor and time.
Besides, if the centers of sensors come off the sides of the web due to vibrations, etc., the operator has to put them back into their places by applying a ruler while the production line is being operated; thus, the position-correcting work takes a great deal of labor and time.